I'd kick 'em in the nutz and say "there's no best scale to play over anything!" Then I'd explain and say, well, there are a few scales that would WORK over this chord, but depending on what you're after, some might be better than others.
The obvious ones are Bb major, Bb pentatonic, gmin. F would also work, it would give an aug4 over the Bb chord. Hell, you could play Cmaj if you wanted. I've never been a fan of just playing scales over a chord unless its for practice. I guess if I HAD to choose, I'd choose F.